Various techniques for preventing howling in a loud speaker system for a lecture, a concert or the like have been provided. In a general howling suppression method, when occurrence of howling is detected, a frequency range causing the howling is attenuated by a filter.
In an audio signal amplifier circuit described in patent document 1, when howling is detected, a low pass filter for removing howling is operated so as to decrease a gain of the audio signal amplifier circuit. The audio signal amplifier circuit checks whether or not the howling is detected by stopping the operation of the low pass filter at every predetermined time interval. The audio signal amplifier circuit decreases the gain in a stepwise manner until the howling is not detected. The audio signal amplifier circuit fixes the gain when the howling is not detected.
Patent document 2 discloses a method in which presence or absence of occurrence of howling is detected based on a frequency characteristic of an input signal and a filter characteristic for suppressing howling is calculated.
Non-patent document 1 proposes a howling canceller with the use of synchronized addition of an M-sequence noise. The howling canceller described in the non-patent document 1 outputs in advance a high level M-sequence noise so as to perform a training of an adaptive filter. The howling canceller outputs the M-sequence noise having a low level during its operation so as to continuously update the adaptive filter. In addition, the howling canceller disclosed in the non-patent document 1 determines that a disturbance level is high and stops the updating of the adaptive filter in a case where an input level of a microphone becomes high, because when the disturbance level becomes high due to continuous speaking of a person or generation of a sound of a musical instrument, the updating of the adaptive filter is inhibited.
Patent Document 1: JP-A-7-15788
Patent Document 2: JP-A-6-327088
Non-patent Document 1: “Inspection relating to removal of howling in an audio system” by Makoto Itami, Mitsutoshi Hatori, Institute of Electronics, Information, and Communication Engineers, Technical Reports EA89-4, in 1989